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University
of Rhode Island GreenShare Factsheets
Petunias
Petunias are popular and reliable flowering ornamental plants.
Versatile annuals, petunias can be used in flower beds, hanging
baskets, window boxes and other types of containers. They also
can be used as cut flowers. The blossoms of petunias come in
many colors with petal edges that are straight or ruffled. Two general classes of petunias are Grandiflora and Multiflora.
Grandiflora petunias are characterized by having fewer, but larger,
showy flowers. A number of Grandifloras are cascade selections,
well suited to growing in hanging baskets, window boxes and other
types of planters. Multiflora petunias have a more compact growth
habit and have smaller but more numerous blossoms. Multifloras
generally withstand wind and hard rains better than the Grandifloras.
Both Grandiflora and Multiflora types may have single flowers,
with one set of petals on each flower, or double flowers with
multiple sets of petals on each flower.
There are hundreds of petunia varieties available in the
nursery industry. Local landscape nurseries and other retail
stores usually handle those varieties that are best suited to
local environmental conditions.
Petunias are normally propagated from seed. Petunia seed
is best sown indoors four to six weeks prior to planting outdoors.
Sow the seed in a moist, well drained, pasteurized medium. Commercially
prepared mixes work best to germinate seed. Do not cover the
seed when sowing.
Water with a fine mist from above or sub-irrigate from below
by setting the germination container in a shallow container of
water. Excessive water should be allowed to drain. The germinating
medium can be kept from drying out by covering the container
with a pane of glass or with a plastic bag. Keep the container
in a location with bright light, but not in direct sunlight,
until germination begins. Maintain a minimum soil temperature
of 70 degrees F and maximum of 80 degrees F. Seedlings can emerge
in three to four days under optimum conditions. During germination,
try to avoid conditions (seedlings being too thick, excessive
soil moisture and cool soil temperatures) that favor damping-off
disease.
Once the seedlings have emerged, remove the covering and
allow the growing medium to become slightly dry between waterings,
but do not allow the seedlings to wilt. Watering is critical
at this small seedling stage! Fertilize the young seedlings with
a diluted formulation of starter fertilizer.
Transplant the seedlings to trays or individual containers
at the two leaf stage, or in about 14 to 21 days. Use disease
free containers and sterile growing medium. Carefully lift the
young seedlings from the germinating medium using a knife or
flat stick. Lift only a few at a time to prevent excessive root
drying. When transplanting, hold the young leaves of the seedlings,
not the stems. Maintain the plants at about 65 degrees F, if
possible. Fertilize every two weeks with one ounce of 20-20-20
fertilizer in three gallons of water. When well-rooted, these
plants can be moved outdoors on warm days. Like purchased transplants,
they should be acclimated slowly to the outdoors. Protect the
seedlings to prevent sunscald, wind damage or wilting.
Petunia plants, whether purchased or grown at home, should
be carefully transplanted when set outdoors. Plant after all
danger of frost is past and when the soil temperature has reached
at least 60 degrees F. Avoid excessive root and soil disruption
around the transplant. Plant the young transplants at the same
depth they were growing in the containers. Water thoroughly after
transplanting to avoid excessive wilting. A soil ridge around
the plant will help hold water in the vicinity of the plant.
Young plants may need protection for a few days following transplanting
during hot, windy weather. Plant on a cloudy day if possible.
Plants can be spaced about 12 inches apart in the garden.
Petunias do best in full sun, although they will tolerate
several hours of light shade a day. Two pounds of 5-10-5 fertilizer
or one pound of 10-10-10 dry fertilizer per 100 square feet can
be incorporated into the soil at planting. A half rate application
can be spread around the plants in July if needed. Too much fertilizer
can cause excessive vegetative growth and sparse flowers. In
high pH soils (alkaline soils) additional iron or iron sulfate
may be beneficial to reduce chlorosis (yellowing) of the foliage.
Water to a depth of six to eight inches when the soil becomes
dry. The frequency of watering will depend on the type of soil,
weather conditions and the amount of mulch. A mulch will not
only reduce soil water evaporation but will also reduce splashing
of water onto the lower leaves, moderate soil temperatures and
reduce weed competition. Do not allow the soil around the plants
to remain excessively wet for several days, as this can lead
to stunted, chlorotic growth or disease. Remove weeds from the
plantings that compete with the petunias for moisture, nutrients
and light.
"Pinching" the plants can be used
to increase the number of flowering stems and discourage excessive
vegetative growth.
Remove old flowers (dead heads) when they start to fade to encourage
repeat blooming.
Petunias are relatively free of disease and insect pests.
Damping-off can be a serious disease problem, however, rotting
the seeds during germination or killing the seedlings after emergence.
Good sanitation practices and maintenance of proper moisture
and temperature levels can minimize damping-off disease.
Petunias with virus diseases can have foliage that is stunted
and deformed, often with light-green streaks, and discolored
and deformed flowers. The best control is to remove and destroy
diseased plants and keep aphids and other insects which transmit
the virus under control. Alternaria blight, crown rot, fusarium
wilt, botrytis and fasciation are other diseases of petunias.
Insects are generally not a problem on petunias. However, isolated
cases of aphid and cutworm infestations
do occur. Remember that healthy, vigorous plants are less susceptible
to pest damage than unhealthy plants.
Yellow foliage on petunias may also be caused by nitrogen
or iron deficiency within the plant. Both can be corrected with
the proper supplemental fertilizers.
Adapted from Dale T. Lindgren,
Nebraska Extension, 2000
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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